Weld thru primer

ABSTRACT

THE PRESENT INVENTION PERTAINS TO A NOVEL COATING SYSTEM FOR APPLICATION TO CARBON STEEL SUBSTRATES. THE COATING PROVIDES EXCEPTIONAL ARE BEHAVIORAL PROPERTIES DURING THE METAL JOINING BY FUSION OR RESISTANCE WELDING OF SUCH SUBSTRATES, VIA ITS METALLURGICAL INFLUENCE ON THE WELD ZONE. THE FUNCTIONAL ELEMENTS OF THIS COATING BASICALLY COMPRISE A CAREFULLY BALANCED MIXTURE OF A SACRIFICIAL OR ANODIC METAL AND A METAL ALLOY SUCH AS CHROMIUM OR NICKEL WHICH SIGNIFICANTLY AFFECTS THE WELDABILITY AND WELD PROPERTIES OF THE COATING. THE POWDERED METALS OR PIGMENTS ARE ATTACHED TO THE CARBON STEEL SUBSTRATES VIA ANY OF A NUMBER OF LIQUID SILICATE BINDERS, THE USE OF WHICH IS WELL ESTABLISHED IN THE PRIOR ART. PREFERRED EMBODIMENTS INCLUDE VARIOUS ADDITIONAL CONSTITUENTS, E.G. MATERIALS SUCH AS BORON OR MAGNESIUM TO INCREASE ROCKWELL HARNESS AND SUSCEPTIBILITY TO HIGH TEMPERATURE CREEP. AMONG THE UNIQUE FEATURES AND ADVANTAGES OF THE PRESENT COATING IS THE REALIZATION OF AN UNUSUAL BALANCE OF FUNCTIONAL PROPERTIES, THE MOST SIGNIFICANT OF WHICH ARE THE BENEFICIAL CONTRIBUTION TO THE METALLURGICAL PROPERTIES OF THE FINISHED WELD, WHILE ACTUALLY IMPROVING THE STABLE ELECTRICAL ARC DISCHARGE NORMALLY ASSOCIATED WITH RESISTANCE WELDING. A FURTHER ADVANTAGE OF THIS COATING CONCEPT IS THAT IS ALLOWS ARRANGEMENT OF SELECTED METALLIC CONSTITUTENTS WITHIN THE COATING IN SUCH A MANNER AS TO ASSURE OPTIMUM SILICATE TO SILICATE BONDING WHEN OVERCOATING WITH A CONVENTIONAL INORGANIC ZINC COATING AFTER FABRICATION OF THE STRUCTURE IS COMPLETED.

United States Patent U.S. Cl. 148--22 12 Claims ABSTRACT OF THE DISCLOSURE The present invention pertains to a novel coating system for application to carbon steel substrates. The coating provides exceptional arc behavioral properties during the metal joining by fusion or resistance welding of such substrates, via its metallurgical influence on the weld zone. The functional elements of this colating basically comprise a carefully balanced mixture of a sacrificial or anodic metal and a metal alloy such as chromium or nickel which significantly affects the weldability and weld properties of the coating. The powdered metals or pigments are attached to the carbon steel substrates via any of a number of liquid silicate binders, the use of which is well established in the prior art. Preferred embodiments include various additional constituents, e.g. materials such as boron or magnesium to increase Rockwell hardness and susceptibility to high temperature creep. Among the unique features and advantages of the present coating is the realization of an unusual balance of functional properties, the most significant of which are the beneficial contribution to the metallurgical properties of the finished weld, while actually improving the stable electrical arc discharge normally associated with resistance welding. A further advantage of this coating concept is that it allows arrangement of selected metallic constituents within the coating in such a manner as to assure optimum silicate to silicate bonding when overcoating with a conventional inorganic zinc coating after fabrication of the structure is completed.

BACKGROUND AND SUMMARY OF THE INVENTION This application is a continuation-in-part of application Ser. No. 873,602, filed Nov. 3, 1969, and now abandoned. The instant invention is related to that field of art concerned with special purpose coating systems for application to carbon steel substrates in order to enhance the fabrication of such substrates by fusion or resistance welding. This invention is concerned with a technique for metallurgical improvement of weld quality via a primer coating system which is engineered to subsequently form alloys during the process of electrical arc-weld deposition. This coating is further designed to receive conventional overcoats, such as metallic zinc, or of passive pigmented intermediate or build coats, finish coatings, or the like.

In order to efliciently meet the demanding corrosion protection requirements of carbon steel surfaces, many synthetic coating systems having an exceptional lifetime have been developed. Optimum performance of these coatings is predicated on the near complete removal of surface contamination and the subsequent formation of a sharp profile or anchor pattern in the steel surface. That is, the surface must be both clean and somewhat rough. This has heretofore been achieved by manually controlled sandblasting equipment after complete assembly of the structure. More recently, it has been recognized that these objectives can be achieved, together with considerable monetary and time savings, and a significant reduction in airborne pollution, by subjecting the surface to an 3,759,756 Patented Sept. 18, 1973 'ice automated shot/ grit blasting and protective coating prior to fabrication. Thus, the cost of removal of spent abrasives as well as contamination problems inherent in conventional sandblasting are eliminated by this automatic blasting. Furthermore, the problem of inaccessible or difficult to reach areas, which cannot be manually blasted after assembly, is circumvented by shot blasting prior to assembly.

In the prior art, the protective coating of a shot blasted surface prior to and during fabrication has been accomplished by conventional steel primers utilizing red lead or chromate inhibitive pigments, and more recently, metallic zinc-pigmented weld thru primers. A number of problems have been encountered upon weld deposition over these conventional primers, which are accented by the so-called weld thru types. First, it has been found that these primers (including the zinc-pigmented weld thru types) produce relatively large amounts of noxious gases K and fumes upon being decomopsed during the welding of the base substrate. Thus, the confined welder has heretofore required special ventilation equipment and/or chemical mask protection such that his ingestion of toxic fumes can be controlled. Second, it has been consistently observed that conventional primers, including the weld thru types, promote arc instability due to their inherent electrical resistance, and/ or the electrical resistance of the by-products of their combustion or burning. Third, certain foreign metallic formations produced during weld deposition tend to poison the weld and thereby drastically increase the probability of defects in the finished weld. These conventional primers have been found to produce impurities as an aftermath of their burning which migrate into the weld base metal and which often have a substantial deleterious effect on its metallurgical properties. This problem is of particular concern in the welding of higher carbon alloy or more complex alloy substrates, and has necessitated the elimination of prior-art coatings from such usage.

The above and other prior-art problems are readily overcome by the novel thin film covering provided by Way of the present invention which favorably influences the metallic structure at the interface (heat affected zone) of the parent metal and the filler metal. This can be achieved while maintaining stable arc initiation and discharge and smooth weld behavior and its attendant effects encountered in the welding of steel substrates that have been pre-coated. Furthermore, this invention provides a coating which constitutes a satisfactory primer for subsequent heavy duty metal/silicate coating systems with, for example, the primers ductile alloying powder constituent achieving a specially shaped non-ferrous metal surface which will afford attractive bonding sites for subsequently applied silicate compounds. An optional mechanical flaking of the alloy powder may be implemented to improve film continuity and coverage of blast produced anchor-profile With a relatively thin film.

The present invention provides a weld thru coating formulation which enhances the subsequent weldability and physical properties of welds applied to a base substrate coated therewith. The coating basically comprises a sacrificial or anodic metal selected from the group of metals having an oxidation potential greater than that of iron; a liquid binder; and a metal-alloy constituent agent. Preferred embodiments include, in addition, metallic adhesion promoting agents, degassing agents, thickening agents, and hydrolytic catalysts, either alone or in combination.

The present invention provides a process for enhancing the subsequent weldability of a steel surface by improving the metallurgical phenomena within the weld zone. Additionally, the generation of toxic and other harmful fumes,

compared to welding of steel surfaces having been primed with conventional coatings, is greatly suppressed. Moreover, these unique metallurgical properties can be imparted to a steel surface coated with the present coating, without deleteriously affecting weld arc stability or weld deposition rates.

DESCRIPTION OF PREFERRED EMBODIMENT The present pre-weld coating basically comprises a sacrificial or anodic metal, selected from the group con sisting of zinc and other metals having an oxidation potential greater than that of iron; a liquid binder; and one or more metal alloy constituents. Preferred embodiments include the addition of other materials such as degassing agents, thickening agents, all as hereinafter defined.

The sacrificial or anodic agent described above must have an oxidation potential greater than +0.44 Eox (value in volts, referred to the hydrogen-hydrogen ion couple as zero) Which is that of iron, the base substrate. Suitable anodic metals, other than zinc, having an oxidation potential greater than iron, are for example: vanadium, manganese, titanium, aluminum, and the like. From a chemical standpoint, as well as for economic reasons, zinc or aluminum is preferred. The anodic agent must be present in a finely divided form, e.g. in a powder or dust state, generally of a particle size between about 2 to microns, preferably presenting an average particle size of about 4 to 7 microns. The anodic element of this invention is generally employed in amounts of about to 75%, preferably about 30 to 50%, by weight of the total of the metallic components in the coating. Commercially pure materials (i.e. 97% or above), as are readily available in the art, are acceptable. Impurities normally present in such powders normally do not have a deleterious effect on the finished coating.

The liquid binder is a silicate material which must be capable of retaining the metal particles in intimate contact with the base substrate after drying (solvent release) or curing. The liquid binder is preferably present in a final weight ratio of about 4 to 19 percent, based on nonvolatiles in the dry film. Suitable silicate binders are the silicates having alkali complexes such as ammonium, sodium, potassium, lithium, non-aqueous ethyl silicates, and related ethoxyethyl esters of silicic acid, and in some cases mixtures thereof. The organic coating vehicles in resinous form cited in prior art technology are generally found to be unacceptable since they provide a source of carbon which is undesirable in the finished weld.

The metallic alloy can comprise any metal or combina-v tion thereof selected from those heavy metals which impart favorable or desirable physical properties to a carbonsteel parent metal. These may be used alone or in combination with each other. Preferred metals are chromium, molybdenum and nickel. These alloy metals in order to be effective, must be incorporated as a substantial percentage of the total metal powder. Generally, effective percentages are found to be from about 25 to 70% by weight of the total metallic pigments. The metallic alloy is present in the mixture in a finely divided state of commutation, generally in a size range between about 2 and microns. Generally, the purity of these metals as supplied commercially is considered suitable for this application.

Another preferred embodiment of the present invention includes the incorporation of a viscosity agent in the present mixture. The viscosity agent minimizes the settling of the dense metallic particles during storage, and enhances the resistance to flow (sagging) after deposition of the film. Exemplary of the viscosity agents that can be employed are zinc stearate, aluminum stearate, organomontmorillonite, flame blown silica and hydrogenated castor oil.

A degassing agent is also preferably incorporated in the present weld thru coating. Suitable materials are silica,

calcium, calcium oxide, tetraethyl ortho silicate, and similar moisture scavenging materials.

In order to fully appreciate the unique features of the aforementioned coating system, one must recognize the relationship between welding properties and corrosion control. Recent research and development in the area of corrosion by metallurgists and chemists has resulted in the use of coatings containing metals with higher oxidation potentials than iron (Fe), to protect carbon steel surfaces from corrosion. This protection is theorized as occurring by a sacrificial action whereby the metal with the higher oxidation potential (anode) in the presence of oxygen and electrolytic solutions is oxidized, releasing electrons, and thereby providing electrochemical corrosion protection for the substrate (cathode) surface. This basic phenomenon has led coating chemists to select elemental zinc or aluminum as the optimum metallic component for sacrificial corrosion protection. It is used in this invention for the same purpose.

One problem encountered when studying the weld behavior of a pure zinc film applied to a steel substrate is the large amount of arc-blow that occurs during welding. While not wishing to be bound by theory, it is theorized that the arc-blow and resultant arc instability during welding over certain sacrificial metallic coatings applied to steel is due to a rapid gaseous release, generated by boiling of metallic components. This gaseous release occurs within a low, narrow temperature range. Both zinc and aluminum exhibit an unusually low boiling point. Welding arc temperatures, often in the region of 2500 F., are sufficiently high to release violent gas pockets as the intense heat of the arc provides a dense source of thermal energy forcing the zinc to become gaseous, instantaneously inducing undesirable dynamics during metal deposition.

In prior art technology, the percentage of zinc in such a coating was determined on the basis of corrosion resistance and electrical conductivity, and consequently amounts of to 98% based on the Weight of total metal powder present were found to be the optimum eifective concentrations. In the present invention, a composition of metals wherein the range is from 30% to 75% by weight of metallic zinc, based on total weight of constituent metals, was explored. The most desirable percentages, considering welding properties alone, varied from 30% to 50% depending on density of the metal alloy constituent and other hereinafter mentioned metallurgical considerations. The upper limit or constraint on the amount of such low boiling elements used in a preweld coating is believed to be based on a phase separation (gas pocket), resulting in some boundary layer formation which opposes the smooth and even deposit of molten filler metal. This has been seen during testing of prior-art coatings whose essential constituent comprises a low boiling metal (i.e. elemental zinc).

The following tests show the corrosion resistant capabilities of the various formulations studied:

CORROSIVE CONDITION [Scale of 0 to 10 with 10=excel1ent and 0=gross failure] The other previously mentioned metals which have been added to the coating are filler metals which have been selected for their high boiling point. These metals adjust the temperature-enthalpy curves such that melting, plastic flow, boiling, and gaseous volatilization of the mixture can be controlled. The use of metals such as nickel (boiling point=5280 F.), silver (boiling point=4010 F.), and chromium (boiling point=4500 F.) will contribute appreciably to the arc stability since they depress the formation of the electrically resistant barrier layer introduced by the zinc. It can be seen that such metals do not'achieve a boiling phase during welding, thereby greatly improving the behavior of the arc. It seems practical to consider this phenomenon much in the same way as a metallurgist employs selected constituents to change the thermal properties of various structural alloys.

An additional feature incorporated in the design of this coating lies in the formulators flexibility to adjust pigments to affect the alloy composition across the weld joint. Conventional techniques for welding allow control of the alloy structure by selection of the desired filler metal. However, control of, or introduction of a varying alloy composition across a given weld joint is outside the realm of existing technology. This invention affords the metallurgist an opportunity to introduce alloy constituents such as boron, chromium, or molybdenum into the transition area i.e., the heat affected zone between filler metal and parent metal. Thereby he can control such phenomena as austenitic grain growth in this most critical weld zone.

Elemental boron in metals has been found to increase weld ductility of alloy steels, particularly those subject to high temperatures. This element may be added to the coatings of the present invention and thus is introduced into the weld zone. The boron inhibits austenitic grain growth which occurs above the critical temperature with the result that there is a significant improvement in the ductility of the present coatings.

Where it is anticipated that the coating is to be used in concert with the welding of medium or high carbon content steel, it has been shown that the metallurgical effect of chromium introduced during welding will improve physical properties of the heat-affected weld zone, such properties being ductility, hardness, and notch sensitivity. By introducing such elements as molybdenum, tungsten, and vanadium (which can prevent formation of carbides in martensitic structure upon tempering--i.e. inside the welding zone), the metallurgist can design weld structures showing greater ductility for a given strength or greater strength for a given ductility than for plain carbon steel. Elements such as chromium, nickel, tungsten, silicon, and molybdenum in the coating can be effective in strengthening the ferrite matrix (i.e. in solid solution).

The metallic Weld-thru primer developed in this invention exhibits a unique weld structure and thus represents a major contribution to welding technology; at the same time the coating does not significantly compromise the desired corrosion resistant properties afforded by prior-art coatings. This is the first time these two functions have been incorporated into a single vehicle.

The following typical test results are presented wherein the coating shown was compounded by Weight measure unless otherwise specified.

Example 1 The coating system was prepared from the designated ingredients using high shear mixing procedure. The prehydrolyzed liquid binder solution is weighed directly into a water jacketed stainless steel tank, equipped with a high energy dispersion mixer. The sacrificial-anode metal is added under slow agitation (approximately 2000 to 3000 surface feet per minute). The metallic alloy agent is next added, the speed of the mixer being regulated so as to maintain a continuous vortex to optimize mixing. Where other additives are to be incorporated (i.e. thickening agent, degassing agent, or the like) they are added in a similar manner. After all such materials are added, the mixer speed is then increased such that the dispersion blade reaches a velocity of approximately 5000 surface feet per minute.

TABLE L-GOMPOSITION OF COATING Parts by Nature Type weight sacrificial/anode metal Zinc dust 540 Liquid binder (silicate type) Ethyl silicate. 635 Metal alloy agent Chromium powd 380 Thickening agent... Colloidal silica- 8 Degassing agent Silica gel "I.

Shear test.Five bars were tested in this experiment. Three bars were coated with the experimental product outlined above. Two were left uncoated. Film thickness on the coated bars was held from 1.0 to 1.5 mils (checked with Nordsen dry film thickness gauge). The bars were assembled and welded such that the fillets were placed in shear when tension was applied to the bar ends. Results are indicated in Table II. The rods employed were type numbers 6010 and 7018 manufactured by the Air Reduction Company (AIRCO) TABLE II Avg.

width Type of Ultimate Kips/ Specimen No. of fille rod load inch 1 (Coated) 2. 204 6010 46. 500 21, 2 (Uncoated) 2. 148 6010 48, 000 22, 300 3 (Coated) 2.078 7018 64, 500 4 (Uncoated) 2. 038 7018 48, 000 23, 600 5 (Coated) 2. 7018 62, 000 29, 200

Through this liimted testing, the welds were observed to be improved by the presence of the coating. Obviously, this phenomena is related to the weld continuity, as no metallurgical effect is likely with this low carbon (A-36) steel. The smooth and even weld deposit simply provided a more sound weld.

Break test.A destructive break test is generally recognized by persons schooled in the art of welding over precoated steel, as a method of determining the amount of porosity introduced into the weld by the coating.

Samples were prepared using three pairs of V2 inch by 3 inch bar stock (5 feet long). Two pairs were blasted and coated with the mixture outlined above, the other pair remained uncoated.

Welds were deposited on both surfaces using three types of AIRCO electrodes, numbers 6010, 6013, and 7018. Twenty inch regions were laid with each type of electrode.

With the exception of welds made with No. 7018 rods, no wormholes were evident in either test panel. Porosity was present to a very slight degree within acceptable limits in welds made on both the coated and uncoated steel plate.

TABLE III lgumber o Specimen Length of Number of porosity number specimen Rod type wormholes indicators 7018 (W0-.. 7 (within 1") Many tiny pinholes.

31 4%" 6010 0 Fine lineart porosl y 34 4 (uncoated)... 6010 0 Do.

Although the 6010 and 6013 welding rods showed no porosity in either case, the welds laid with 7018 rods on coated plates had less pinholes than were apparent on uncoated plates.

Toxicity test-The coating described above was further tested to determine a quantitative breakdown of fumes generated during welding. It demonstrated that the fumes released were well below the allowable limits published by the American Conference of Government Industrial Hygienists. (Note: previous testing had indicated that certain conventional pigmented coatings would exceed these limits.)

Tests were conducted using a Gelman #GM-4 membrane filter taped inside the welders helmet. This filter has a 0.8 micron pore size, retaining any particle above 0.3 micron. A flow rate of to 12 liters of vapor per minute (approximate human breathing rate) was drawn through the filter. The sample was drawn throughout the duration of Welding (approximately 20 inches length; 10 minute sample time). The welding electrode was type No. 6010.

Fume samples were analyzed spectrographically to determine the amount and concentration of metallic elements.

Example H The coating system was prepared from the designated ingredients using high shear dispersion equipment normally familiar to paint manufacturers. The liquid binder solution is weighed into a water jacketed stainless steel tank. 0.1 N HCl is slowly added and the resulting mixture agitated for no less than four (4) hours at low speed. At this time, the thickening and anti-mud cracking agents are added and dispersed at high speed (5000 surface feet per COMPOSITION OF COATING Parts] Nature Type weight sacrificial/anode metal- Zine dust 500 Liquid bind Cellosolve silicate 680 Metal/alloy agent Chrome flake 363 Modified hydrogenated 25 Anti-mud eraekin agent Tri methyl borate 13.8 Hydrolyzing agent. 0.1 N H01 (one tenth normal hydro- 23. 8

chlorie acid in water).

1 Cellosolve silicate is a trade name for a commercial preparation of ethoxyethyl silicate.

The coating described above was applied to two pieces of /1 plate-6" wide x longV grooved 60 included angle, thickness on face side and /a groove on backside. A pair of identical steel plates were similarly prepared but left uncoated.

The plate pairs were welded using a shielded metal arc-7/32 AWS wire #E6011 manufactured by Airco. The welds were examined for welding defects by subject ing the bars to a standard Bend Test consisting of bending a bar of the welded material in the region of the weld zone such that the ends were parallel to each other and at a distance apart of 2 /2". Examination of the welded regions yielded the results shown in Table IV.

The test bars were also sectioned and three test specimens were removed for Charpy Impact Testing, using standard test specimens 10 mm. square. The results of the weld testing is described in Table IV.

TABLE IV Specimen number Bend test Charpy V notch 1-77. 0 Coated T-l No visible defects"-.- 2-71. 0 3-77. 0 7-60. 0 Uneoated T-Z One 1/16 crack 875. 0 9-76. 0

Example III The coatings described in Example II were applied at the recommended film thickness and compared to a prior art weldable coating, whose metallic constituents comprised essentially metallic zinc with 5% metallic aluminum at the same film thickness.

The coatings were applied to metal plates x 6" x l5"-60 included angle, V grooved. These plates were welded using a shielded metal are 7/32 AWS wire #E6011. During weld deposition the prior art coating was noted to produce irregularity with a slight tendency to fingernail during deposition of weld metal. No such unstable weld arc behavior was reported in the coating prepared per Example III.

Four specimens were removed from each test bar and examined using the photo-macrographic technique.

In an analysis of approximately four linear inches of weld (selected at random), the prior art coating demonstrated 13 evidences of fine to medium porosity whereas a similar region of weld applied over the coating described herein exhibited only three fine porosity indications.

Example IV The coating system outlined below was prepared from the designated ingredients using high speed shear mixing procedure. The xylene and amine-treated clay are weighed into a water jacketed stainless steel tank, equipped with a high energy dispersion mixer and dispersed at high speed (approximately 5000 surface feet per minute) for 15 minutes. The speed is then reduced to approximately 2000 surface feet per minute and the Cellosolve acetate, zinc dust, chrome flake are mixed with one fourth of the light binder. The hydrolyzing agent required for the entire batch of binder is added slowly and stirring is continued for 20 minutes after this addition. Next the neutralizing and anti-mud cracking agents are added. Slow agitation is continued for four hours. Meanwhile, in a separate tank, the remaining three quarters of the binder and the modified hydrogenated castor oil are dispersed at high speed for approximately 20 minutes. At the end of the four hour stirring period, the pro-thickened Cellosolve silicate is added to the metallic mixture. A final dispersion at 5000 surface feet per minute is made in order to obtain a grind of 4 Hegman.

A control sample of the mixture is then evaluated for viscosity, weight per gallon, and drying time. Where required, the viscosity and weight per gallon are adjusted by the addition of a solvent of the ester family.

COMPOSITION OF COATING Parts Nature Type weighf Solvent Xylene 252. 5

Do- T Cellosolve acetate 252. 5 Solvent thickening agent Amine treated cla 57 sacrificial/anode metal. Zine dust. 500 Metal/alloy agent Chrome fiake 363 Liquid binder Cellosolve silicate 680 Hydrolyz ng agent... 0.1 N H01 30.4 Neutralizing agent A 1% solution of amine N- 14. 0

methyl-diethanol in Cellosolve acetate. Anti-mud cracking agent Trimethyl borate 14. 0 Blnder thickening agent Modified hydrogenated castor oil. 25

1 Cellosolve acetate is a trade name for a commercial preparation of ethylene glycol mono-ethyl ether acetate.

Shear test-Bars coated with the product outlined above were assembled and welded such that fillet welds could be broken apart in order to examine the weld for porosity and evidence of weld defects. In an analysis of approximately 30 inches of A weld bead, only scattered fine porosity as is normal in hand welding was observed.

Example V Adhesion of subsequent coatings-To determine the adhesion properties of the weld-thru primer described herein, three 4" x 8" 14 ga. metal panels were sandblasted to white metal using fine silica sand. Each of the proprietary coatings of Examples II and IV and a commercially available inorganic silicate (2 package type) was applied, one coating to each panel. Coating thickness in each case varied from 0.75 to 1.0 mils dry. The coatings were allowed to cure 24 hours before topcoating.

The commercially available zinc coating was then applied to each panel at a film thickness of 2 to 2 /2 mils dry. The adhesion was tested as described in Federal Standard Test Method 141 #E6301. Results of this testing are reported in Table IV.

It is claimed:

1. A pre-weld coating composition which enhance the stability of the electrical arc discharge during the welding of metals and improves the physical properties of the heat-affected weld zone comprising:

(a) an anodic metal selected from the group of metals having an oxidation potential greater than that of iron,

(b) a metal alloy selected from the group consisting of chromium, molybdenum and nickel, and

(c) a liquid silicate binder material capable of retaining the metal components of the composition in inti mate contact with a base substrate after drying.

2. The composition of claim 1 wherein the anodic metal is present in an amount of about 30 to 75% by weight and the metal alloy is present in an amount of about 25 to 7o% by weight, based on the total metallic components in the coating.

3. The composition of claim 1 wherein the anodic metal and the metal alloy are each present in a finely divided form, the anodic metal having a particle size of about 2 to 25 microns and the metal alloy having a particle size of about 2 to microns.

4. The composition of claim 1 wherein the binder material is selected from the group consisting of ammonium silicate, sodium silicate, potassium silicate, lithium silicate, ethyl silicate, and ethoxyethyl silicate.

5. The composition of claim 4 wherein the binder material is present in an amount of about 4 to 19% by weight based on nonvolatile components remaining after drying.

' 6. The composition of claim 1 wherein the anodic metal is zinc.

7. The composition of claim 1 further including a thickening agent.

8. The composition of claim 7 wherein the thickening agent is selected from the group consisting of zinc stearate, aluminum stearate, organo-montmorillonite, flame-blown silica and hydrogenated castor oil.

9. The composition of claim 1 further including a degassing agent.

10. The composition of claim 9 wherein the degassing agent is selected from the group consisting of silica, calcium, calcium oxide and tetraethyl ortho silicate.

11. The composition of claim 1 wherein the metal alloy comprises a finely divided chromium powder.

12. The composition of claim 11 wherein the metal alloy has been altered to yield a flake-like shape.

References Cited UNITED STATES PATENTS 3,118,048 1/1964 Fisher 219-92 3,175,991 3/1965 Levine 117-131 3,339,058 8/1967 Todd 1l7--160 3,469,071 9/1969 Feldt 1l7135.1

L. DEWAYNE RUTLEDGE, Primary Examiner P. D. ROSENBERG, Assistant Examiner U.S. Cl. X.R. 

